1. Field of the Invention
The present invention relates to the field of radio-frequency power amplifiers. More particularly, the invention relates to a method and apparatus for regulating the supply voltage of a power amplifier operating under large peak-to-average ratios.
2. Background
Dynamic regulation of amplifiers' supply voltage is particularly required in envelope restoration systems and in high-efficiency modulation schemes such as collector, drain or anode modulation, and it may also be used in radar or other pulse amplifier systems. Such regulation of power supply is also required in applications which require generation of pulses with complex envelope modulation patterns, and in optical systems wherein there is a need to modulate an optical channel or an optical transmitter according to a predetermined modulation law.
Efficient amplification of RF signals having varying envelope patterns and peak to average ratios of about 8–11 dB is a common problem in cellular communication systems. Typical RF Amplifiers (RFAMP) are usually not efficient under these operating conditions, mostly due to the low value of the average amplified signal power and due to design requirements of keeping sufficient headroom for amplifying signal peak leads by a larger amplifier with greater DC consumption.
An example of a typical single-ended RF amplifier is depicted in FIG. 1. The notation used in this example, and in other examples to follow, utilize lower case letters to designate variables that are functions of time (the time parameter t is omitted), e.g., e=e(t), and upper case letters to designate constant values e.g., E. For the purpose of simplicity, the Input RF match and the Output RF match components of the amplifier are generally shown as blocks 11 and 12 of the RFAMP 10 in FIG. 1.
Some of the terms that are used herein will now be explained for the sake of clarity. In general, RF signals may be presented as the amplitude-phase modulation of an RF tone. The term video envelope is used herein to refer to the amplitude modulation component of the amplified RF signal. The term video current is used herein to refer to the video envelope of the current flowing though the amplifier's active element (e.g., RF transistor) while it amplifies an RF signal. The term video match generally refers to circuitry that accepts the video current and delivers power supply voltage to the amplifier's active element accordingly.
The supply voltage ed=E+ud is applied to the drain of the RF transistor 13 shown in FIG. 1, at a contact point which is generally referred to herein as “power supply input”, where ud is the varying (alternating) component and E is the constant component of the supply voltage ed. For example, in a classical RF amplifier, fluctuations of the voltage ud are not desirable and thus made as low as possible by an appropriate design of the drain video match circuit 12. The drain current id=I0+iΔ, consumed by the RF transistor 13 during signal amplification, includes a constant component I0, and a varying component (video envelope current) iΔ which is an alternating component having a zero average value i.e., having positive and negative values. It is assumed that RF currents and voltages do not have considerable influence on the values of id and ed, due to an appropriate RF filter (not shown) inside the RFAMP 10, such that their influence may be neglected.
One of the known solutions for the problem described above, which aims to improve RF amplifiers' efficiency, is performing dynamic regulation of the supply voltage of the RF amplifier, and thereby adjusting the supply voltage to be proportional to the envelope of the amplified signal. Usually, a fixed constant voltage E is applied when the magnitude of the video envelope of the amplified signal is lower than a predetermined threshold level. In this way, the consumed DC power is decreased while its linearity is kept at an acceptable level by choosing an appropriate value for the supply voltage E. When the envelope magnitude is larger than the predetermined threshold level, an additional positive voltage ud is added to the supply voltage E. The application of this additional voltage is also known as “voltage enhancement”.
Since the consumed DC power is mostly increased due to the amplifying of the video envelope peaks, that are usually not so frequent, efficiency may be improved by utilizing proper dynamic regulation of the supply voltage ed=E+ud.
The main problem in dynamic regulation of the supply voltage ed(t) is to perform this regulation efficiently and rapidly, since the supply voltage ed(t) should be regulated at real time, and kept in proportions with the video envelope of the amplified signal. For example, it is a difficult task to achieve a response delay of ed(t) which is less than 10 ns with bandwidths of about 50MHz utilizing a typical balanced cellular amplifier having a peak DC power consumption of about 400W. State of the art DC-DC converters, that potentially may provide a convenient solution, cannot so far satisfy the above mentioned response delay time and bandwidth requirements.
Some of the prior art solutions, for example WO95/34128 or WO01/67598, are based on variations of the following scheme. A fraction of an extra fixed supply voltage required is dynamically added to the main supply voltage, and in this way the supply voltage of the RF amplifier is kept in proportions with the video envelope of the amplified signal. In other words, there is a fast and powerful video envelope pulse amplifier having its output combined with a constant supply voltage of an RF amplifier.
However, there are some disadvantages associated with such methods mainly since fast and efficient analog combining of power supplies is difficult to achieve, and since the constant voltage of the extra supply sources sets a limit to the maximal possible voltage enhancement that can be obtained. Moreover, the analog pulse amplifier demands very fast, powerful and thus expensive transistors to meet the low response delay requirements.
On the other hand, binary switching between power supplies does not enable to achieve linear dynamic regulation of the supply voltage, which preferably should be proportional to the video envelope of the amplified signal. Another problem associated with binary switching solutions is that rapid binary voltage and current fluctuations at the drain of the RF transistor spoils the linearity of the amplifier.
Another difficulty associated with power supply regulation implementations is to ensure that the undesirable voltage fluctuations of ud, obtained due to the fluctuations of the video envelop current iΔ, are kept as low as possible. In most cases this is achieved by providing a good ground for variable video envelope current iΔ, when there are no voltage enhancement of ed. Obviously, once such good grounding is achieved the spectral purity of the amplified signal will be significantly improved.
All the methods described above have not yet provided satisfactory solutions for improving the efficiency and performance of dynamically regulated supply voltage of power amplifiers.
It is an object of the present invention to provide a method and apparatus for regulating the supply voltage of an amplifier without carrying out summation of voltage sources.
It is another object of the present invention to provide a method and apparatus for regulating the supply voltage of an amplifier in proportions with the envelope of the amplified signal and with minimal delay.
Other objects and advantages of the invention will become apparent as the description proceeds.